HFC-125 is a useful compound as an alternative chlorofluorocarbon (or flon) compound free of chlorine and is used as a cooling medium, a blowing agent, an injecting propellant and so on. HFC-125 is conventionally produced through a fluorination process of tetrachloroethylene. In such a production process, dichlorotetrafluoroethane, dichlorotrifluoroethane, hexafluoroethane, CFC-115 and so on are produced as by-products.
Among those by-products, CFC-115 has a boiling point of -38.7.degree. C. which is close to a boiling point of HFC-125 of -48.5.degree. C. Further, a value of the relative volatility between these two compounds is close to 1. Particularly, when a mixture contains HC-125 at a concentration of not less than 95 mole % (that is, the concentration of CFC-115 is not more than 5 mole %), the relative volatility value is about 1.04. Accordingly, when the above mixture is distilled to separate HFC-125 at a high concentration through a conventional distillation treatment, a distillation apparatus having much plate numbers is required, which generally means that the separation using distillation is extremely difficult.
In the present specification, when a solution essentially comprising of at least a remarked component A and a remarked component B (a boiling point of component A&lt;a boiling point of component B) is in a vapor-liquid equilibrium state with the vapor thereof, the term "relative volatility (.alpha.)" is defined as follows: EQU .alpha.=(Y.sub.A /X.sub.A)/(Y.sub.B /X.sub.B)
wherein, X.sub.A is a molar fraction of the lower boiling component A in the liquid phase, X.sub.B is a molar fraction of the higher boiling component B in the liquid phase, Y.sub.A is a molar fraction of the lower boiling component A in the vapor phase which is in equilibrium with its liquid phase, and Y.sub.B is a molar fraction of the higher boiling component B in such vapor phase.